Device, method and system for measuring the distribution of selected properties in a material

ABSTRACT

The present invention relates to a device for measuring the distribution of selected properties of materials, said device comprises an emitter of electromagnetic radiation and furthermore at least one sensor of a first type. The emitter emits electromagnetic radiation in a selected frequency range towards said materials and a sensor of the first type detects electromagnetic radiation in a selected frequency range coming from said materials. The detected electromagnetic radiation having been emitted by said emitter. The device also comprises means to generate a three dimensional image contour information regarding the said material&#39;s position in space, and an analyser which (a) receives information from said sensors and (b) processes this information and (c) generates signals containing information about the distribution of said properties as output. The invention also relates to a system and a method for measuring the distribution of selected properties of materials.

FIELD OF INVENTION

[0001] This invention relates to a device for measuring the distributionof selected properties in a material, and in particular a device thatnon-contacting and non-destructively measures the spatial distributionof material properties, such as density, water contents and temperatureof materials, by detecting electromagnetic radiation. The invention alsorelates to a method and a system.

BACKGROUND OF THE INVENTION

[0002] Many industrial processes depend on the measurement of materialproperties as temperature, water contents and material density. A closemonitoring of these material properties results often in increasedefficiency and improved product quality. Additional benefits are likelyto occur, if such measurements can be accomplished fast and in anon-destructive, non-invasive, and non-contacting way with acceptableaccuracy.

[0003] As an example the determination of the temperature distributionin foodstuff during heating process. Here, an on-line monitoring of thetemperature distribution helps to avoid cold spots where bacteria arenot eliminated completely or to reduce the overdue heating time spent toensure complete bacteria elimination. This results in a reduced heatingtime and reduced energy consumption as well as in an increasedthroughput of the production line.

[0004] Material properties are traditionally measured by some form ofdestruction (sample separation, peeking) but can often be measured bythe analysis of transmitted electromagnetic radiation by evaluating thedielectric response of the material. Measurements using electromagneticradiation are generally contact-free and non-destructive.

[0005] A suitable frequency region of electromagnetic radiation todetermine material properties as temperature distribution, watercontents and density is the lower microwave region where waterabsorption is not too large and the wavelength is already short enoughto ensure reasonable spatial resolution. The determination of the abovematerial properties is achieved by analysing the dielectric response ofthe material based on the material's polarisability. Dielectric data ofa material sample are typically obtained in analysing theelectromagnetic wave's reflection and transmission properties or acombination of both. In order to obtain a distribution of the materialproperties, a three-dimensional image of the material's dielectricresponse must be measured. This requires to move the microwave detectorsetup and the material sample relative to each other.

[0006] Prior art instruments make use of either a single measurementfrequency or the emission frequency is swept within a frequency interval(FMCW) and the average delay time is calculated from the obtained data.

[0007] Prior art that approaches to dielectric imaging, use thetransmission of electromagnetic radiation of a single frequency (or asmall band) between a multitude of antenna locations or the materialsample is shifted and rotated or shifted in two dimensions in order toobtain a spatial resolution. Based on these data the dielectric image isobtained e.g. by the well-known CSI (contrast source iteration) methodwhere the location and strength of polarisation sources arc obtained inan iterative process.

[0008] Using techniques well-known to a skilled person (i.e. ContrastSource Iteration CSI, as described below) the electromagnetic picture isused to calculate the unknown dielectric functions in the dielectricpicture.

[0009] Starting from Maxwell's Equations (as described by R. F.Harrington, in the book with the title “Time Harmonic ElectromagneticFields”, published by Mc. Graw Hill 1961) one assumes that any regionwhere the dielectric function is different from unity, theelectromagnetic field creates bound charges due to polarisation. Thesebound charges are created by the electric field itself and theyoscillate with it resulting in an additional current component:

j(p)=∈(p)·u(p)

[0010] where the current density is j, the electric field is u and thedielectric function of the material is ∈ and of the background isdenoted ∈_(b). Assume p and q to be two position vectors in a twodimensional cross section of the measurement gap. D is a domain whichcontains the cross section of the material sample. The vector q denotesthe source point of the electromagnetic radiation. Based on that ageneral relation for the connection between the electric fields in themeasurement space is obtained formally by applying the definition of aGreen's function for the electric current:${u_{j}(p)} = {k^{2}{\int\limits_{D}{{G\left( {p,q} \right)} \cdot {j(q)} \cdot {{v(q)}}}}}$

[0011] Inserting the above current density relation and splitting theintegral yields:${u_{j}(p)} = {{k^{2}{\int\limits_{D}{{G\left( {p,q} \right)} \cdot ɛ_{b} \cdot {u(p)} \cdot {{v(q)}}}}} + {k^{2}{\int\limits_{D}{{G\left( {p,q} \right)} \cdot \left\lbrack {{ɛ(p)} - ɛ_{b}} \right\rbrack \cdot {u(p)} \cdot {{v(q)}}}}}}$

[0012] Here the first term denotes the electric field when thedielectric response of the background is present only, the second termstands for the fields generated by polarisation i.e. a dielectriccontrast. The fields when only a background is presented are referred toas incident fields u^(inc). Then the field at an observation pointincident from the radiation source is (according to an article by P. M.van den Berg, B. J. Kooj, R. E. Kleinman, with the title “ImageReconstruction from Iswich-Data III”, published in IEEE Antenna andPropagation Magazine, Vol.41 No.2 April 1999, p.27-32):${u_{j}(p)} = {{u_{j}^{inc}(p)} + {k^{2}{\int\limits_{D}{{G\left( {p,q} \right)} \cdot {\chi (q)} \cdot {u_{j}(q)} \cdot {{v(q)}}}}}}$

[0013] where G denotes the two-dimensional Green's function of theelectromagnetic problem${G\left( {p,q} \right)} = {\frac{i}{4}{H_{0}^{(i)}\left( {k \cdot {{p - q}}} \right)}}$

[0014] and the polarisability function χ depends on the dielectricfunction of the material ∈ and the background ∈_(b) in the followingway: ${\chi (p)} = \frac{{ɛ(p)} - ɛ_{b}}{ɛ_{0}}$

[0015] Defining scattered fields f one obtains directly:${F_{j}(r)} = {{{u_{j}(r)} - {u_{j}^{inc}(r)}} = {k^{2}{\int\limits_{D}{{G\left( {r,q} \right)} \cdot {\chi (q)} \cdot {u_{j}(q)} \cdot {{v(q)}}}}}}$

[0016] From this an integral equation for the scattered electric fieldat any point r is set up.${F_{j}(r)} = {\frac{i}{4}k^{2}{\int\limits_{D}{{{H_{0}^{(i)}\left( {k \cdot {{r - q}}} \right)} \cdot \chi}{(q) \cdot \left\lbrack {{F_{j}(q)} + {u_{j}^{inc}(q)}} \right\rbrack \cdot {{v(q)}}}}}}$

[0017] This relation is fulfilled exactly when r is equal to the antennalocation and the F_(i)(r) are measured values of the scattered fieldsfor a given wave vector k for a frequency f:$k = {\frac{2\pi}{c} \cdot {f.}}$

[0018] The values of Fi (r) for the points interior to the region D areonly fulfilled approximately. So the above relation has to be solved fora set K of k vectors and a set Q of internal points resulting in a[K·Q]×[K·Q] non-linear matrix problem for the fields F_(i)(r) and thepolarisabilities χ(r).

[0019] In matrix form the state equation becomes:

u=u ^(inc) +Gχu

[0020] whereas the frequency relation is:

F=Gχu

[0021] Introducing the contrast source φ=χ·u the above relations become

[0022] φ=χu^(inc)+χGφ at all Q interior points, for any of the Kmeasurement frequencies

[0023] F=Gφ at a single antenna location, for any of the K measurementfrequencies

[0024] Using the method of conjugated gradients sequences for thecontrasts and the contrast sources solving the above problem areobtained.

OBJECTIVE OF THE INVENTION

[0025] A device has been designed to measure the spatial distribution ofthe temperature, water contents and density distribution in a materialbased on the dielectric and magnetic information contained intransmission measurements obtained using microwave radiation.

[0026] This invention covers two methods to resolve such informationfrom measured data:

[0027] (a) The temperature, density and water contents profile can beobtained by interpolation between a set of previously measured materialsamples where the profiles are known in advance. There the measurementresult is found by a best fit to the interpolation database.

[0028] (b) The said profile is found by direct calculation of theinverse scattering problem resulting in a known distribution of thedielectric and magnetic properties. Based on models on the dependence ofthe dielectric and magnetic properties as functions of the wantedparameters, a map of the said properties is obtained directly.

[0029] The instrument proposed here may only use one mechanical scanningdimension. Due to the usage of a multi-channel antenna and a multitudeof frequencies, a two-dimensional cross-section of the dielectricpicture is obtained. This calculation process involves a novel methodrelated to contrast source iteration where the location and strength ofpolarisation sources are obtained in an iterative process based ontransmitted electromagnetic field measurements at a multitude offrequencies. Thereby the antenna patterns must be frequency dependentand they are assumed to be directed in cross section of the sampleallowing an essentially two dimensional approach.

[0030] In order to facilitate the calculation of the dielectricparameters, regions where the dielectric properties are at first orderconstant are obtained by an e.g. evaluating video pictures taken from atleast two different points of view with overlapping image region. Fromthese video pictures a reasonable guess of the material sample'sdielectric structure is made. As an alternative ultrasound images can beused for the same purpose or a three dimensional image of the materialsample may be stored in a memory.

SUMMARY OF THE INVENTION

[0031] The object with the invention is thus to provide a device thatmeasure the spatial property distribution in a non-contacting andnon-destructively way.

[0032] In accordance with the invention this object is achieved by thefeatures in the characterising portion of claim 1.

[0033] This object is also achieved by the features in thecharacterising portion of claim 12 and claim 17.

[0034] An advantage of the invention is that it provides on-line fastmeasurements of spatially resolved material parameter distributions bymeans of a combined application of microwave reflection and transmissionmeasurements and a three dimensional contour of the material.

[0035] Traditionally the temperature and density of the material samplesis obtained by probing a certain fraction of the material samples. Thismethod allows a complete on-line monitoring of all material samples inproduction increasing the degree of product control.

[0036] The accuracy of the measurement is checked by means ofcalibration samples with known constituents and known temperatureprofile which are measured at regular intervals. Thereby it issufficient to perform invasive temperature measurements after the samplehas been measured at different points of the sample and compare them tothe instrument's findings. As a additional verification process the sameprocedure can be repeated when the sample has e.g. cooled down.

[0037] Summarising the advantages of this invention it provides anon-destructive, non-invasive and non-contacting, fast and automaticmeasurement process of the water contents and temperature distributionof dielectric bodies requiring minimal human intervention. Themeasurement process is insensitive to changes in product size, form andpositioning. Other features of the current invention will become moreapparent in the following detailed description of the preferredembodiment which by means of example illustrate the principles of thisinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0038]FIG. 1 is a schematic diagram of a first embodiment of a deviceaccording to this invention.

[0039]FIG. 2 is a chart indicating a dielectric model for chickenanticipated for reduction of the amount of unknown variables of thesample's dielectric behaviour.

[0040]FIG. 3 is a chart indicating a dielectric model for breadanticipated for reduction of the amount of unknown variables of thesample's dielectric behaviour.

[0041]FIG. 4 illustrate a cross section of a bread loaf, where thedielectric model from FIG. 3 is mapped.

[0042]FIG. 5 is a schematic chart indicating the evaluation process inorder to obtain moisture, density and temperature data from dielectricproperties.

[0043]FIG. 6 is a schematic diagram of a second embodiment of a deviceaccording to this invention.

[0044]FIG. 7 is a flow chart of the whole calculation process.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0045] As shown in FIG. 1 the primary elements of a measurement device10 according to a first embodiment of the present invention are amicrowave generator 11, a transmitting antenna 12, a receiving antenna13, an analyser 14. These elements work together to analyse thedistribution of material properties (such as water contents, density andtemperature) in a material sample 16. The sample is carried on aconveyor means 17, which may consist of a slide table mounted on alinear motor, and is arranged in a measurement gap between saidtransmitting antenna 12 and receiving antenna 13.

[0046] The generator 11 is connected to the transmitting antenna 12 andgenerates electromagnetic radiation, which is transmitted from thetransmitting antenna 12 towards the receiving antenna 13. The materialsample 16 is placed between said transmitting antenna 12 and saidreceiving antenna 13, which indicate that at least a part of thetransmitted radiation passed through the material sample 16. Theelectromagnetic radiation is transmitted in the form of signals 18, eachhaving a first amplitude and phase, and a different frequency within afrequency range.

[0047] The generator 11 is also connected to the analyser 14, andinformation regarding the amplitude and frequency of each transmittedsignal 18 is sent to the analyser 14.

[0048] The transmitted signals 18 pass, at least partially, through thematerial sample 16 and are received by the receiving antenna 13 asreceiving signals 19 each having a second amplitude and phase, which maybe different from the first amplitude and phase, for each differentfrequency.

[0049] The receiving antenna 13 is connected to the analyser 14, whichreceives information regarding the received signals 19. The analyser 14compares the amplitude and phase of the transmitted signal with thecorresponding amplitude and phase for the received signal, for eachtransmitted frequency.

[0050] Each transmitting antenna 12 is designed to emit electromagneticradiation of a set of selected frequencies partially impinging on andflowing through the material samples 16. Each receiving antenna 13 isdesigned to receive electromagnetic radiation emitted from any transmitantenna 12 and at least partially transmitted and reflected by thematerial sample 16. The receiving antenna 13 may be set up at one ormore positions enabling to scan the material sample 16.

[0051] The analyser 14 acts as interface between the raw data and theuser. The output of the analyser 14 consists of a three-dimensionalpicture of the material sample's properties as density, water contentsand/or temperature.

[0052] Information about the microwave attenuation and runtime (or phaseand damping of the microwave power wave) between the transmittingantennas 12 and receiving antennas 13 are calculated in the analyser 14.For each frequency of the chosen frequency set and for a chosen set oftransmitting-/receiving antenna pair and at a fixed point on thematerial sample 16 such a calculation is performed.

[0053] In this embodiment of the invention it is assumed that the shapeof the material sample 16 is known, and a three dimensional image of thematerial sample is stored in a memory 15 connected to the analyser 14.The three dimensional image may be used to calculate cross-sectionalimages for each measurement position of the material sample on theconveyor means 17. Examples of a material where the three dimensionalimage is known are fluids passing through the gap in a tube or sampleshaving a defined shape, such as candy bars.

[0054] For all measurement positions along the material sample 16, theresults of the damping and phase measurement, for all frequencies, areused to determine an electromagnetic picture, which is obvious for aperson skilled in the art, and since this is not an essential part ofthe invention these steps are not disclosed in this application. Theposition information from the memory is saved as a three dimensionalsurface position data set describing the three dimensional contour ofthe material sample 16.

[0055] The material properties (such as water contents, density andtemperature) in a material may be obtained by interpolation of thematerial property distributions in the following.

[0056] Assume a set of material samples has been measured previously asreferences. The data sets are stored in their original size or in atransformed form to reduce the data size. For these materials, thedistribution of the parameters to be measured is known. These can bedifferent temperatures, different temperature profiles, differentdensity and water contents distributions. Extracted parameters of themeasurement of these reference products form a point in a highdimensional vector space. To each point in this space a specificdistribution of the parameters to be determined is associated byinterpolation of the adjacent points of the reference measurements. Themeasurement results on an unknown product is now associated with anotherpoint on this vector space. Since the parameter distribution to bemeasured is known for a certain region in the vector space, thedistribution associated with the measured point yields the measurementresult.

[0057] On the other hand direct calculation of the material propertydistribution may be applied.

[0058] Together with a three dimensional model of the dielectricstructure of the material sample this three dimensional picture is usedto determine regions within the measurement gap where the (yet unknown)dielectric function of the material can be assumed non-changing. FIG. 2illustrates a model for chicken 20 and FIG. 3 illustrates a model forbread 30.

[0059] Each model comprises several regions 21, 31, where the dielectricfunction is assumed to be constant. The number of regions in the modelsmay be adjusted, even during the process of obtaining the materialproperties, to obtain a smooth, but not too smooth, curve for thedielectric constant as a function of x and y co-ordinates, ∈(x,y).

[0060] The regions in FIG. 3 are divided by concentric circles 32 and anumber of mapping points P1-P14 are arranged on the outer concentriccircle 33. The distance between each mapping point is preferablyessentially equal.

[0061] The appropriate model is adapted to the three dimensional imageof the sample material, in this example a bread loaf. FIG. 4 illustratesa cross-section 40 of a three dimensional image of the bread loaftogether with an x-axis and an y-axis. The contour of the bread isindicated by the line 41, which is derived from the three dimensionalsurface position data set stored in the memory, and the mapping pointsP1-P14 in FIG. 3 are mapped upon the contour line 41. The concentriccircles 32 in FIG. 3 are adjusted after the shape of the contour whichis illustrated by the lines 42 in FIG. 4 divides the cross section ofthe bread loaf into regions 43 where the dielectric constant is assumedconstant.

[0062] Below is described a simplified approach of CSI, anticipatingregions where the dielectric function is constant, as indicated in themodels described in FIG. 2 and 3.

[0063] Starting with the relation between the scattered field at a givenlocation as a function of the contrast source one can simplify thesolution process considerably when the location of regions where thedielectric function is constant are known a priori : $\begin{matrix}{{u_{j}(p)} = {{u_{j}^{inc}(p)} + {k^{2}{\int\limits_{D}{{G\left( {p,q} \right)} \cdot {\chi (q)} \cdot {u_{j}(q)} \cdot {{v(q)}}}}}}} \\{{u_{j}(p)} = {{u_{j}^{inc}(p)} + {k^{2}{\int\limits_{D}{{G\left( {p,q} \right)} \cdot {\chi (q)} \cdot {u_{j}(q)} \cdot {{v(q)}}}}}}} \\{= {{u_{j}^{inc}(p)} + {k^{2}\underset{{n = 1}\quad}{\overset{N\quad}{\sum\quad}}{\chi_{n} \cdot {\int\limits_{D_{N}}{{G\left( {p,q} \right)} \cdot {u_{j}(q)} \cdot {{v(q)}}}}}}}}\end{matrix}$

[0064] where G denotes again the two-dimensional Green's function of theelectromagnetic problem${G\left( {p,q} \right)} = {\frac{i}{4}{H_{0}^{(i)}\left( {k \cdot {{p - q}}} \right)}}$

[0065] and the polarisability χ_(n) depends on the dielectric functionof the material ∈ being constant on the region D_(m) and the background∈_(b) in the following way: $\chi_{n} = \frac{ɛ_{n} - ɛ_{b}}{ɛ_{0}}$

[0066] Obviously the above step reduce the matrix size from the numberof contrast sources to the number of different regions taken intoaccount.

[0067] From the above a similar integral equation for the scatteredelectric field at any point r is set up.${F_{j}(r)} = {\frac{i}{4}k^{2}\underset{{n = 1}\quad}{\overset{N\quad}{\sum\quad}}{\chi_{n} \cdot {\int\limits_{D_{n}}{{H_{0}^{(i)}\left( {k \cdot {{r - q}}} \right)} \cdot \left\lbrack {{F_{j}(q)} + {u_{j}^{inc}(q)}} \right\rbrack \cdot {{v(q)}}}}}}$

[0068] For this relation a similar solution process as in the generalcase is applied:

[0069] Below is described a calculation of the dielectric function forone pair of antennas for various frequencies for frequency independentpolarisation.

[0070] Starting with the relation between the scattered field at a givenlocation as a function of the contrast source one can simplify thesolution process considerably when the location of regions where thedielectric function is constant are known a priori :${u\left( {p,f} \right)} = {{u^{inc}\left( {p,f} \right)} + {k^{2}{\int\limits_{D}{{G\left( {p,q,f} \right)} \cdot {\chi (q)} \cdot {u\left( {q,f} \right)} \cdot {{v(q)}}}}}}$

[0071] In a step similar to the above procedure, the relation issimplified by introducing regions where the dielectric function isassumed to be constant:${u_{j}\left( {p,f} \right)} = {{u^{inc}\left( {p,f} \right)} + {k^{2}\underset{{n = 1}\quad}{\overset{N\quad}{\sum\quad}}{\chi_{n} \cdot {\int\limits_{D_{N}}{{G\left( {p,q,f} \right)} \cdot {u\left( {q,f} \right)} \cdot {{v(q)}}}}}}}$

[0072] where G denotes again the two-dimensional Green's function of theelectromagnetic problem${G\left( {p,q,f} \right)} = {\frac{i}{4}{H_{0}^{(i)}\left( {k \cdot {{p - q}}} \right)}}$

[0073] and the polarisability χ_(n) depends on the dielectric functionof the material ∈ being constant on the region D_(m) and the background∈_(b) in the following way: $\chi_{n} = \frac{ɛ_{n} - ɛ_{b}}{ɛ_{0}}$

[0074] The wave vector k is defined to be the wave propagation constantin the background medium given by ∈_(r,b), μ_(r,b):

k=2πf{square root}{square root over (∈₀μ₀∈_(r,b)μ_(r,b))}

[0075] From the above a similar frequency dependent integral equationfor the scattered electric field at any point r is set up.${F\left( {r,f} \right)} = {\frac{i}{4}k^{2}\underset{{n = 1}\quad}{\overset{N\quad}{\sum\quad}}{\chi_{n} \cdot {\int\limits_{D_{N}}{{H_{0}^{(i)}\left( {k \cdot {{r - q}}} \right)} \cdot \left\lbrack {{F\left( {q,f} \right)} + {u^{inc}\left( {q,f} \right)}} \right\rbrack \cdot {{v(q)}}}}}}$

[0076] For this relation a similar solution process as in the generalcase is applied.

[0077] Below is described a calculation of the dielectric function forone pair of antennas for various frequencies for frequency dependentpolarisation.

[0078] A first order approximation for the frequency dependence of thepolarisation is obtained by grouping the measurement frequencies in twogroups, a group at lower and a group at higher frequencies. The abovesummarised calculation process is repeated twice and the difference inthe obtained polarisation values gives a measure for its frequencydependence.

[0079] In order to calculate the material parameters based on dielectricdata, the relation between the material parameters as density,temperature and water content is needed. For most applications thefollowing model for the temperature dependence of the dielectricfunction of water (extracted from experimental data published in IEEEPress 1995 by A. Kraszewski, with the title “Microwave Aquametry”) is:$\begin{matrix}{{ɛ_{H2O}(T)} = \frac{ɛ_{\infty}(T)}{1 + {\omega^{2}{\tau (T)}}}} & (1)\end{matrix}$

[0080] An approach (based on a simple volumetric mixing relation yieldsthe dielectric chart depicted in FIG. 5 where the real and imaginaryparts of the dielectric function are taken as independent co-ordinates:

∈(T,c _(H2O) ,d)=(1−c _(H2O))·∈_(basis) ·d+c_(H2O)·(∈_(H2O)(T)−∈_(basis) ·d)  (2)

[0081] Obviously every point in the complex dielectric plane stands fora unique water contents and material temperature when the dielectricproperties of the dried base material do not change considerably. Anunique density temperature plot is obtained, when the water contents isuniform.

[0082] From the spatial distribution of the dielectric function of thematerial sample 16, its density distribution moisture content andtemperature are readily obtained applying a water model (see equation 1)and a mixing relation (see equation 2). This part of the evaluation isshown schematically in FIG. 5, a schematic view of the completecalculation process is given in FIG. 7.

[0083] The imaginary part of the dielectric constant Im(∈) forms a firstaxis in FIG. 5 and the real part of the dielectric constant Re(∈) formsa second axis, perpendicular to the first axis. The real part ispositive and the imaginary part is negative. Any material without watercontent have a specific dielectric constant, so called ∈_(dry), whichvary between point 50 and 51 depending on the material, both only havinga real part. On the other hand, pure water having a temperature of 4° C.has a dielectric constant 52 comprising both a real part and animaginary part, and when the temperature of the water increase itfollows a curve 53 to a point where pure water has a temperature of 99°C. and a dielectric constant 54. The real part of the dielectricconstant for materials containing any amount of water decreases withhigher temperature and the imaginary part of the dielectric constant formaterials containing any amount of water increases with highertemperature. For illustration see the dashed lines in FIG. 5 for watercontent of 25, 50 and 75%.

[0084] An example of a dielectric value 55 is indicated in FIG. 5. Thevalue 55 is situated within a region 56 delimited by the curve 53,stretching between point 52 and 54, a straight line between point 54 and∈_(dry) and a straight line between ∈_(dry) and point 52. As mentionedbefore, if the temperature increase, with constant water content, thevalue of the dielectric constant 55 moves to the left in the graph asindicated by the arrow 56, and if the temperature decrease, withconstant water content, the value 55 moves to the right as indicated bythe arrow 57. On the other hand, if the water content decrease, withconstant temperature, the value 55 moves towards ∈_(dry) as indicated bythe arrow 58, and if the water content increase, 25 with constanttemperature, the value 55 moves away from ∈_(dry) as indicated by thearrow 59.

[0085] For each defined region 43 the calculated, or estimated,dielectric constant may be directly transformed into water content andtemperature.

[0086]FIG. 6 illustrates a measurement device 60 according to a secondembodiment of the present invention. This embodiment comprises the sameparts as the first embodiment described in connection with FIG. 1,except that the memory 15 is replaced with a video imaging arrangementcomprising two video cameras 61 and 62, both connected to an evaluationunit 63, which in turn is connected to the analyser 14.

[0087] Each video camera 61, 62 continuously take pictures of thematerial sample 16. The pictures are sent to the evaluation unit 63,where a three dimensional picture is created using known techniques. Theresulting three dimensional picture similar to the one that was storedin the memory 15 in the first embodiment.

[0088] By using video imaging the system gets more flexible and it ispossible to use the measurement device on material samples having anunknown shape or even a changing shape depending the water contentand/or the temperature.

[0089] In the above evaluation the major reason to use video imaging isto reduce the number of unknowns in the calculation process to obtainthe dielectric function's distribution in the material sample. Theobtained reduction in calculation time is necessary (at least in today'savailable calculation power) to speed up the measurement process. Inthis preferred embodiment, the material samples are easily accessible tovideo imaging. If this is not the case, alternative solutions areultrasound imaging. If the material samples have a simple geometric formor if subsequent material samples are very similar, no extra imaging isnecessary to perform the above calculation process as described in theFIG. 1.

[0090] The calculation of the dielectric image (of a two-dimensionalcross section) of the material sample in the measurement gap isaccomplished by solving the previously described inverse scatteringproblem.

[0091] Both video cameras 61 and 62 image the part of the measurementgap. The location of the cameras 61 and 62 are chosen in a way to enablethe reconstruction of a three-dimensional picture where the materialsample 16 is positioned within the measurement gap.

[0092] For each position of the material sample 16 a three-dimensionalpicture of the sample location in the measurement gap is calculatedbased on images taken by the video cameras 61 and 62.

[0093] In addition the position information contained in the opticalimage is used together with a priori knowledge of the material structurethe obtain a first guess of the dielectric structure under measurement.This enables to reduce the number of unknowns of the dielectric imagingcalculation process drastically (about two orders of magnitude) and tospeed up the calculation considerably.

[0094]FIG. 7 show a schematic view of the complete calculation processfor the device according to the invention.

[0095] As previously described in connection with FIG. 1 and FIG. 6, theinput data to the analyser comprises the microwave transmissionmeasurements, i.e. information regarding the emitted signals 18(amplitude and phase for each used frequency) and the detected signals19 (amplitude and phase for the corresponding frequency). Thisinformation is input in the calculation process, 71.

[0096] Information regarding the image contour of the material sample 16is also needed and inputted into the process, 72. A predeterminedresolution of the image contour is used to start the calculationprocess. The resolution may be increased or decreased dependent on thecalculation results, as described below. Information regarding theposition of the material sample 16 in the measurement gap is alsoinputted into the process in 72.

[0097] The information from 72 is used to establish an object geometry,73. A model, for instance as described in FIG. 3, is thereafter used todetermine regions wherein the dielectric function is assumed of thefirst order, i.e. constant. The number of regions used is set in themodel. The selected model, in this case model 30, is used to establishregions in the material sample 1G by adjusting the concentric circles tothe result of the object geometry from 73, which is done in 74, asdescribed in FIG. 4.

[0098] The geometry assumptions from 74 and the result from themicrowave transmission measurement from 71, is thereafter used tocalculate the dielectric constant within each region, 75. Thecalculation process have previously been described in this application.

[0099] Another piece of information is needed to convert the dielectricconstant into water content and/or temperature, that is the dielectricconstant for the material sample 16, when there is no water content inthe material, ∈_(dry). This information may be obtained from literatureor from previously made measurements on similar material samples, 76.

[0100] This information is used to establish the equations defining therelation between the dielectric constant and the water content andtemperature, as described in connection with FIG. 5, 77.

[0101] The resulting dielectric constant within each region from 75 isthereafter translated (or converted) into water content and temperature,78.

[0102] If the calculation process is well established the followingsteps may be unnecessary, but in most cases they are necessary to avoidunreasonable results.

[0103] In 79, a check is made to determine if the resulting temperatureand water contents are reasonable, i.e. the temperature is greater thanzero, T>0, the water content is greater than zero, C_(H20)>0 (i.e.Im(∈)<0) and if the water content is less than 100%, C_(H20)<100%.

[0104] If any of the above mentioned checks does not pass, thecalculation process is fed back via 80, where the position of thematerial sample is updated. If video cameras are used, as described inFIG. 6, a new image contour of the material sample is used to repeat thesteps 74, 75 and 78. In the case where the image contour information ispreviously stored in a memory, as described in FIG. 1, the calculationprocess may make a small adjustment to the material size, deform thematerial contour, translate the material in one direction and the repeatsteps 74, 75 and 78.

[0105] If no objections are raised regarding reasonable results in 79the process continue to 81, where the smoothness of the curve describingthe dielectric function across the cross section of the material sampleis investigated. If the dielectric function is too smooth or not enoughsmooth, the process is fed back via 82, where the resolution of theimage contour is changed. Thereafter the steps 73, 74, 75 and 78 arerepeated before the checks 79 and 81 are performed again.

[0106] There is also a possibility to change the number of regions inthe used model in 74 to increase or decrease the number of regions tocalculate.

[0107] If the smoothness of the curve is acceptable, the process proceedto 83, where a new dry dielectric constant, ∈_(dry), of the material iscalculated depending on the calculated results in the process. If thecalculated dry dielectric constant, ∈_(dry), does not correspond withthe used dry dielectric constant, ∈_(dry,prior), the dielectric constantis updated in 84 and the translation of the dielectric function in step76, 77 and 79 are repeated, before the checks 79, 81 and 83 areperformed again.

[0108] If no objections are raised in 83, the process present theresults in the form of water content and or temperature.

[0109] The calculation process described in FIG. 7 is normally performedfor a position of the material sample in the measurement gap. When thecalculation process is completed the conveyor means 17, on which thematerial sample 16 is moved to a new position where another measurementis performed. The updated information, regarding, ∈_(dry), number ofregions, position of material, and so on, are used at the next positionto speed up the process.

[0110] In a further embodiment of the present invention a multiple ofreceiving antennas may be used to allow a single processing as describedin FIG. 7, to establish the three dimensional temperature, or watercontent, distribution within the material sample.

[0111] The calculation process in FIG. 7 only describe the embodimentwhere the model is used to establish regions, where the dielectricconstant is assumed constant.

1. A device (10; 60) for measuring the distribution of selectedproperties of a material (16) arranged on a conveyor means (17), saiddevice comprises an emitter (12) of electromagnetic radiation arrangedat one side of said conveyor means (17), said emitter emitselectromagnetic radiation (18) in a multiple of frequencies in aselected frequency range towards said material (16), at least a sensor(13) arranged on an opposite side of said conveyor means (16), comparedto said emitter (12), said sensor (13) detects electromagnetic radiation(19) in said selected frequency range being emitted from said material(16), said electromagnetic radiation originating from said emitter (12),and an analyser (14) is arranged to receive information regarding saidemitted electromagnetic radiation (18) and said detected electromagneticradiation (19), said information comprises amplitude and/or phase foreach selected frequency, characterised in that said device furthercomprises: an image device (15; 61-63) which is arranged to sendinformation to said analyser (14), to create a three dimensional contourof the material, said analyser (14) is arranged calculate the selectedproperty distribution in said material (16) based on the receivedinformation.
 2. The device according to claim 1, wherein said imagedevice is a memory (15), having stored information regarding the threedimensional contour of the material (16).
 3. The device according toclaim 1, wherein said image device comprises at least an imaging sensor(61, 62) is connected to an image processing device (63), said at leastone imaging sensor (61, 62) each detects an image of said material,which is processed into a three dimensional contour of said material insaid image processing device (63).
 4. The device according to claim 3,wherein said at least one imaging sensor (61, 62) detects a picture ofthe reflectivity in optical wavelengths.
 5. The device according toclaim 4, wherein said at least one sensor of a second type is a videocamera (61, 62).
 6. The device according to any of claim 3, wherein saidimaging sensor detects a picture of the reflectivity and transmissivityand propagation speed of sound waves.
 7. The device according to claim6, wherein said at least one sensor of a second type is an ultrasoundimaging device.
 8. The device according to any of claims 1-7, whereinsaid analyser (14) is provided with means to interpolate previouslymeasured results, stored in a memory, to obtain the selected propertydistribution in said material (16).
 9. The device according to any ofclaims 1-7, wherein said analyser (14) is provided with means tocalculate the dielectric distribution in said material (16) and convertsaid dielectric distribution into the selected property distribution insaid material (16).
 10. The device according to claim 9, wherein saidmeans to calculate the dielectric distribution comprises a threedimensional model (20, 30) determining regions within said material (16)where the dielectric function is assumed non-changing, and means toapply said model (20, 30) to said three dimensional contour of thematerial, whereby a dielectric distribution is obtained.
 11. The deviceaccording to claims 9 or 10, wherein said device is provided with meansto convert said dielectric distribution into the selected propertydistribution.
 12. A method for measuring selected properties of amaterial (16) arranged on a conveyor means (17), said method comprisingthe steps: (a) emitting electromagnetic radiation (18) in a selectedfrequency range towards said material (16) from an emitter (12) arrangedat one side of said conveyor means (17), (b) detecting electromagneticradiation (19) in said selected frequency range in a sensor (13)arranged on an opposite side of said conveyor means (16), compared tosaid emitter (12), said electromagnetic radiation (19) being emittedfrom said material (16), said electromagnetic radiation (19) originatingfrom said emitter (12), (c) transmitting information, comprisingamplitude and/or phase for each selected frequency, regarding saidemitted electromagnetic radiation (18) and said detected electromagneticradiation (19) to an analyser (14), characterised in that said methodfurther comprises the steps: (d) transmitting information, comprisinginformation to create a three dimensional contour of the material, tosaid analyser (14), and (e) calculating the selected propertydistribution in said material (16) by analysing said information in theanalyser.
 13. The method according to claim 12, wherein the step ofcalculating the selected property distribution comprises the step ofinterpolating previously measured results, stored in a memory.
 14. Themethod according to claim 12, wherein said step of calculating theselected property distribution comprises the steps: calculating of thedielectric distribution in said material using the information regardingemitted (18) and detected (19) electromagnetic radiation, and convertingthe dielectric distribution into the selected property distribution insaid material (16).
 15. The method according to claim 14, wherein saidstep of calculating the dielectric distribution comprises the steps:providing a three dimensional model (20, 30) determining regions withinsaid material (16) where the dielectric function is assumednon-changing, and applying said model (20, 30) to a three dimensionalcontour obtained by an imaging device (15; 61-63).
 16. A systemcomprising a device according to any of claims 1 to 11, said instrumentevaluates the measurement data based on a method according to any ofclaims 12 to 15 in order to obtain information on the spatialdistribution of the said dielectric and magnetic properties of saidmaterials, which information is used to calculate the temperature,density and/or water contents distribution of said materials.